The present invention relates to a method for preparing formic acid, the method being based on preparing methyl formate with methanol as the reactant and on acid-catalyzed hydrolysis of methyl formate, which is a so-called equilibrium reaction. Methyl formate can be prepared at the beginning of the process from carbon monoxide and methanol. In addition to formic acid, the hydrolysis of methyl formate produces methanol which is circulated back to the preparation of methyl formate.
The hydrolysis of methyl formate is normally carried out in a pressure reactor on the basis of homogeneous acid catalysis, wherein an acid solution is used as a catalyst and/or the autocatalytic nature of the hydrolysis reaction is utilized; that is, the produced formic acid is used as the catalyst. To obtain sufficient conversion, the hydrolysis is carried out at a temperature of about 120xc2x0 C. and under a pressure of about 10 bar. At such high temperatures, a high pressure is necessary to prevent evaporation of the reaction mixture, because the hydrolysis reaction will only take place in the liquid phase.
The reaction mixture is led into a distillation column where the mixture of methyl formate and methanol is separated from the mixture of water and formic acid. Methyl formate and methanol are separated from each other in a separate methyl formate circulation column, and they are both returned to the process. Formic acid is concentrated in several successive distillation columns, and the separated water is circulated back to the hydrolysis stage (Hase, A., Koppinen, S., Riistama, K., Vuori, M.: Suomen kemianteollisuus, pp. 53-54, Chemas Oy, 1998).
The majority of the costs in the above-presented process are caused by energy consumption at the distillation stages (water separation) and the reverse reaction of formic acid to methyl formate and water during the first distillation stage. The conversion of methyl formate obtained in the process is low, due to the equilibrium nature of the reaction and the reverse reaction.
In principle, the above-mentioned costs can be reduced either by producing more concentrated formic acid at the hydrolysis stage and/or by preventing the reverse reaction of formic acid. The hydrolysis of methyl formate as an equilibrium reaction at different temperatures and under different pressures is well known, and consequently it is not possible to affect the economy of the process by varying the temperature and the pressure to a great extent.
The separation of reaction products from each other in a reaction mixture has been examined in an annular chromatographic reactor filled with activated carbon (Cho, B. K.: Studies of Continuous Chromatographic Reactors, Dissertation Thesis, University of Minnesota, 1980) and in a conventional tubular chromatographic reactor filled with activated carbon (Wetherhold, R. G., Wissler, E. H., Bischoff, K. B.: An Experimental and Computational Study of the Hydrolysis of Methyl Formate in a Chromatographic Reactor, Chemical Reaction Engineering, 1974: 133, 181-190) by using as an eluent 0.5 to 1.0 M hydrochloric acid HCI which also acts as a catalyst in the reaction. With low feeding concentrations of methyl formate, it was possible to achieve conversions of even 100%, but in this case also the concentration of produced formic acid remained relatively low in view of its economic exploitation.
In this way, separation and concentration of methanol was achieved in both cases, but concentration of formic acid could be attained only in the latter reactor. However, this application involves the problem that the acid catalyst must be separately added into the system and separated after the reaction.
Also known is the hydrolysis of methyl formate by using a heterogeneous catalyst. Thus, a solid catalyst is used as the catalyst instead of an acid solution. In practice, this means a porous, strongly acidic polystyrene divinyl benzene (PS-DVB) based cation exchange resin (EP patent 596 484, SU inventor""s certificate 1085972) with sulphonic acid as the functional group. It is possible to use both a macroporous and a gel-like cation exchanger. In this application, a weakly acidic cation exchanger was not found to have the catalytic property.
Using a heterogeneous catalyst, the advantage is achieved that the catalyst can be easily separated from the reaction mixture after the reaction. Such a hydrolysis can be carried out both in a batch-type stirred and pressure reactor. The separation of formic acid can be performed by conventional distillation after the reaction from the whole reaction mixture or in connection with the reaction (reactive distillation).
However, it is not possible in a stirred reactor or a conventional pressure sure reactor to utilize the property of the ion exchange resin to separate the reaction products, and thus no conversions greater than the equilibrium conversion are achieved. This property is based on the fact that different compounds are adsorbed with different affinities onto the surface of various adsorbent materials. For example, activated carbon and strongly acidic cation exchange resin have very different adsorption properties with respect to the compounds in question.
The final result of the hydrolysis stage, particularly the formic acid concentration, is significantly affected by the composition of the reaction mixture at the initial stage. Depending on the implementation of the process, the water/methyl formate ratio varies to a great extent. It can be for example 0.3:3, 14:1, 1:1.5, 1.5:1, or 1:1.
Now, it has been observed that it is possible to reduce both of the above-mentioned costs (energy consumption in water separation by distillation and the reverse reaction of formic acid to methyl formate) simultaneously by shifting the equilibrium of the reaction towards the reaction products, i.e. formic acid and methanol. In practice, this means separation of the reaction products from each other in the reaction mixture and concentration of these components. This is possible in a chromatographic reactor, which is a certain type of a tubular reactor and which is filled with a suitable solid material. The reactant mixture, containing methyl formate and water, is supplied into the reactor, and a reaction mixture is received from the reactor. This mixture usually contains unreacted reactants in addition to the reaction products.
It is an aim of the present invention to simultaneously utilize not only the catalyzing property of a strongly acidic ion exchange resin but also its property to separate different components of a reaction mixture by performing the hydrolysis of methyl formate in a chromatographic reactor. In a chromatographic reactor (column), it is possible to utilize this property of the catalyst to separate different components and thus to achieve higher conversion of methyl formate than the equilibrium conversion of the reactant feed, on one hand by preventing the reverse reaction of formic acid and methanol and on the other hand by shifting the reaction towards the products by removing product components from the reaction mixture.
Furthermore, the above-described separation of the catalyst material needed in the stirred reactor is not necessary, because the reaction solution flows through a solid, stationary bed of the ion exchange resin. In the solution according to the invention, it is also noteworthy that high conversions can be achieved even at room temperature.
By carrying out the hydrolysis in a continuously operating multicolumn system, it is possible to achieve greater conversion, better separation of the reaction components, more economic use of different solutions, and thereby better productivity of the process. In such a system, the points of inlet and outlet of different flows are moved from one ion exchange bed to another in such a way that separate compounds are obtained as the output, particularly formic acid, in as pure and concentrated continuous product flows as possible.
In the solution according to the invention, the expensive and complex separation of reaction products, which is typical for processes of prior art, is simplified and facilitated, and the energy consumption of the separation stage can be reduced. Furthermore, the energy consumption of the separation stage can be further reduced by using instead of water one of the reactants as an eluent in the system, namely methyl formate, wherein the amount of water in the reaction mixture can be considerably reduced.